Make better use of i18n (#20096)

* Prototyping

* Start work on creating offsets

* Modify tests

* Start prototyping with actual MPH

* Twiddle around

* Twiddle around comments

* Convert templates

* Fix external languages

* Fix latest translation

* Fix some test

* Tidy up code

* Use simple map

* go mod tidy

* Move back to data structure

- Uses less memory by creating for each language a map.

* Apply suggestions from code review

Co-authored-by: delvh <dev.lh@web.de>

* Add some comments

* Fix tests

* Try to fix tests

* Use en-US as defacto fallback

* Use correct slices

* refactor (#4)

* Remove TryTr, add log for missing translation key

* Refactor i18n

- Separate dev and production locale stores.
- Allow for live-reloading in dev mode.

Co-authored-by: zeripath <art27@cantab.net>

* Fix live-reloading & check for errors

* Make linter happy

* live-reload with periodic check (#5)

* Fix tests

Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: zeripath <art27@cantab.net>

templates/user/dashboard/feeds.tmpl

@@ -114,7 +114,7 @@
 					<p class="text light grey">{{$.i18n.Tr "action.review_dismissed_reason"}}</p>
 						<p class="text light grey">{{index .GetIssueInfos 2 | RenderEmoji}}</p>
 					{{end}}
-					<p class="text italic light grey">{{TimeSince .GetCreate $.i18n.Lang}}</p>
+					<p class="text italic light grey">{{TimeSince .GetCreate $.i18n}}</p>
 				</div>
 			</div>
 			<div class="ui two wide right aligned column">

templates/user/dashboard/milestones.tmpl

@@ -91,7 +91,7 @@
 								</div>
 							</div>
 							<div class="meta">
-								{{ $closedDate:= TimeSinceUnix .ClosedDateUnix $.i18n.Lang }}
+								{{ $closedDate:= TimeSinceUnix .ClosedDateUnix $.i18n }}
 								{{if .IsClosed}}
 									{{svg "octicon-clock"}} {{$.i18n.Tr "repo.milestones.closed" $closedDate|Str2html}}
 								{{else}}