Refactor i18n to locale (#20153)

* Refactor `i18n` to `locale`

- Currently we're using the `i18n` variable naming for the `locale`
struct. This contains locale's specific information and cannot be used
for general i18n purpose, therefore refactoring it to `locale` makes
more sense.
- Ref: https://github.com/go-gitea/gitea/pull/20096#discussion_r906699200

* Update routers/install/install.go

templates/repo/commits.tmpl

@@ -10,7 +10,7 @@
 					<span class="text">
 						{{svg "octicon-git-branch"}}
 					</span>
-					{{.i18n.Tr "repo.commit_graph"}}
+					{{.locale.Tr "repo.commit_graph"}}
 				</a>
 			</div>
 		</div>