Sign merges, CRUD, Wiki and Repository initialisation with gpg key (#7631)

This PR fixes #7598 by providing a configurable way of signing commits across the Gitea instance. Per repository configurability and import/generation of trusted secure keys is not provided by this PR - from a security PoV that's probably impossible to do properly. Similarly web-signing, that is asking the user to sign something, is not implemented - this could be done at a later stage however.

## Features
- [x] If commit.gpgsign is set in .gitconfig sign commits and files created through repofiles. (merges should already have been signed.)
- [x] Verify commits signed with the default gpg as valid
- [x] Signer, Committer and Author can all be different
    - [x] Allow signer to be arbitrarily different - We still require the key to have an activated email on Gitea. A more complete implementation would be to use a keyserver and mark external-or-unactivated with an "unknown" trust level icon.
- [x] Add a signing-key.gpg endpoint to get the default gpg pub key if available
    - Rather than add a fake web-flow user I've added this as an endpoint on /api/v1/signing-key.gpg
    - [x] Try to match the default key with a user on gitea - this is done at verification time
- [x] Make things configurable?
    - app.ini configuration done
    - [x] when checking commits are signed need to check if they're actually verifiable too
- [x] Add documentation

I have decided that adjusting the docker to create a default gpg key is not the correct thing to do and therefore have not implemented this.

public/less/_base.less

@@ -950,6 +950,22 @@ footer {
     }
 }
 
+i.icons .icon:first-child {
+    margin-right: 0;
+}
+
+i.icon.centerlock {
+    top: 1.5em;
+}
+
+.ui.label > .detail .icons {
+    margin-right: 0.25em;
+}
+
+.ui.label > .detail .icons .icon {
+    margin-right: 0;
+}
+
 .lines-num {
     vertical-align: top;
     text-align: right !important;

public/less/_repository.less

@@ -1212,6 +1212,15 @@
             border-bottom-left-radius: 0;
         }
 
+        &.isSigned.isWarning {
+            border: 1px solid #db2828;
+            background: fade(#db2828, 10%);
+
+            .detail.icon {
+                border-left: 1px solid fade(#db2828, 50%);
+            }
+        }
+
         &.isSigned.isVerified {
             border: 1px solid #21ba45;
             background: fade(#21ba45, 10%);