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.

modules/git/utils.go

@@ -7,6 +7,7 @@ package git
 import (
 	"fmt"
 	"os"
+	"strconv"
 	"strings"
 	"sync"
 )
@@ -86,3 +87,30 @@ func RefEndName(refStr string) string {
 
 	return refStr
 }
+
+// ParseBool returns the boolean value represented by the string as per git's git_config_bool
+// true will be returned for the result if the string is empty, but valid will be false.
+// "true", "yes", "on" are all true, true
+// "false", "no", "off" are all false, true
+// 0 is false, true
+// Any other integer is true, true
+// Anything else will return false, false
+func ParseBool(value string) (result bool, valid bool) {
+	// Empty strings are true but invalid
+	if len(value) == 0 {
+		return true, false
+	}
+	// These are the git expected true and false values
+	if strings.EqualFold(value, "true") || strings.EqualFold(value, "yes") || strings.EqualFold(value, "on") {
+		return true, true
+	}
+	if strings.EqualFold(value, "false") || strings.EqualFold(value, "no") || strings.EqualFold(value, "off") {
+		return false, true
+	}
+	// Try a number
+	intValue, err := strconv.ParseInt(value, 10, 32)
+	if err != nil {
+		return false, false
+	}
+	return intValue != 0, true
+}