Fixes composer warnings when: a) mentioning groups ("By mentioning @group, you are about to notify x people...") and b) mentioning users in a PM ("You mentioned @user but they won`t be notified...")
Ember3 is more picky about having a container being destroyed and it's easier to cause exceptions, especially in tests.
This fix has been initially created for an exception occuring in tests when running discourse-code-review and discourse-polls tests at the same time. `getCurrentUser` method body was failing as the container was destroyed.
Original stacktrace:
```
"Error: Assertion Failed: expected container not to be destroyed
at new EmberError (ember:2929:31)
at assert (ember:1793:23)
at Container.lookup (ember:17736:64)
at PluginApi.getCurrentUser (discourse/lib/plugin-api:56:31)
at allowUser (javascripts/discourse/initializers/init-code-review:38:29)
at eval (javascripts/discourse/initializers/init-code-review:78:11)
at eval (select-kit/mixins/plugin-api:86:19)
at Array.forEach (<anonymous>)
at eval (select-kit/mixins/plugin-api:85:44)
at Array.forEach (<anonymous>)"
```
Somehow a plugin or some new Chrome bug is causing its heuristic to detect
our textarea for the composer as a target for address autocomplete
This is likely a chrome bug but this change is very safe regardless.
New `about.json` fields (all optional):
- `authors`: An arbitrary string describing the theme authors
- `theme_version`: An arbitrary string describing the theme version
- `minimum_discourse_version`: Theme will be auto-disabled for lower versions. Must be a valid version descriptor.
- `maximum_discourse_version`: Theme will be auto-disabled for lower versions. Must be a valid version descriptor.
A localized description for a theme can be provided in the language files under the `theme_metadata.description` key
The admin UI has been re-arranged to display this new information, and give more prominence to the remote theme options.
