Improve UI for becoming a candidate for existing pools (#2620)

1. If an address has been a pool some time ago, but was deactivated since,
   we can't use addPool() method anymore due to limitations in the contract.
   Instead, we fetch previously existing pool, provide unchangeable mining
   address, and use stake() method for reactivating the pool.

2. Banned pools can't become a candidate again, so the button is disabled.

apps/block_scout_web/assets/js/pages/stakes/become_candidate.js

@@ -52,7 +52,11 @@ async function becomeCandidate ($modal, store, msg) {
       return false
     }
 
+    if (msg.pool_exists) {
+      makeContractCall(stakingContract.methods.stake(store.getState().account, stake.toString()), store)
+    } else {
       makeContractCall(stakingContract.methods.addPool(stake.toString(), miningAddress), store)
+    }
   } catch (err) {
     openErrorModal('Error', err.message)
   }

apps/block_scout_web/lib/block_scout_web/channels/stakes_channel.ex

@@ -77,6 +77,7 @@ defmodule BlockScoutWeb.StakesChannel do
   end
 
   def handle_in("render_become_candidate", _, socket) do
+    pool = Chain.staking_pool(socket.assigns.account)
     min_candidate_stake = ContractState.get(:min_candidate_stake)
     token = ContractState.get(:token)
     balance = Chain.fetch_last_token_balance(socket.assigns.account, token.contract_address_hash)
@@ -85,12 +86,15 @@ defmodule BlockScoutWeb.StakesChannel do
       View.render_to_string(StakesView, "_stakes_modal_become_candidate.html",
         min_candidate_stake: min_candidate_stake,
         balance: balance,
-        token: token
+        token: token,
+        pool: pool
       )
 
     result = %{
       html: html,
-      min_candidate_stake: min_candidate_stake
+      balance: balance,
+      min_candidate_stake: min_candidate_stake,
+      pool_exists: not is_nil(pool)
     }
 
     {:reply, {:ok, result}, socket}

apps/block_scout_web/lib/block_scout_web/templates/stakes/_stakes_modal_become_candidate.html.eex

@@ -14,7 +14,14 @@
             </div>
           </div>
           <div class="form-group">
-            <input mining-address type="text" class="form-control" placeholder="<%= gettext("Your Mining Address") %>">
+            <input
+              mining-address
+              type="text"
+              class="form-control"
+              placeholder="<%= gettext("Your Mining Address") %>"
+              value="<%= @pool && @pool.mining_address_hash %>"
+              <%= if @pool do "disabled" end %>
+            />
           </div>
           <p class="form-p m-b-0">Minimum Stake:
             <span class="text-dark">

apps/block_scout_web/lib/block_scout_web/templates/stakes/_stakes_top.html.eex

@@ -12,7 +12,12 @@
             <%= render BlockScoutWeb.StakesView, "_stakes_btn_remove_pool.html", text: gettext("Remove My Pool"), extra_class: "js-remove-pool" %>
           <% end %>
         <% else %>
-          <%= render BlockScoutWeb.CommonComponentsView, "_btn_add_full.html", text: gettext("Become a Candidate"), extra_class: "js-become-candidate" %>
+          <%=
+            render BlockScoutWeb.CommonComponentsView, "_btn_add_full.html",
+              text: gettext("Become a Candidate"),
+              extra_class: "js-become-candidate",
+              disabled: @account[:pool] && @account.pool.is_banned
+          %>
         <% end %>
       </div>
     </div>