Core updates now support !!! (important)

There have been some small internal API changes that have been causing grief for some users. While all updates are technically "safe", user workarounds and custom implementations can never be predicted with certainty. This change will allow us to say with confidence, either

- Yep, no worries this is a safe update. Relax. Versus;
- Might want to watch this one, just in case.

Merge pull request octobercms#2784 from acasar/patch-2

Fix translatable URL caching

Added a check for missing dependencies in two places

- System dashboard now lists missing dependencies
- Installing a plugin with missing dependencies will also attempt to install those dependencies at the same time
Fixes octobercms#36

Specify explicit form element

This allows request elements external to the form to serialize a target form for use. An example might be a search form at the top of a page, list of results in the middle, then the pagination at the bottom. The pagination would need to target the search form with data-request-form to persist the search query value.

This should work with the JS API too, passed as an object, because $($()) is acceptable in jQuery

API docs progress

Controller -> SystemController for consistency

Rollback 84b5334

See octobercms#2749, octobercms#2750 & octobercms#2751 on notes for how to better handle this. Put simply, the control of disabled VS previewMode should be given to the individual form widget, as opposed to the spawning parent.

Menu items are supposed to be counted in 100s

Plugin authors may need to adjust their menu items from this.

Prefer to keep this language key unique

Just in case we decide to call it something else on the dashboard
Revert octobercms#2709 (aec4591)

System files should return fully qualified path

- Not sure how this was not fixed sooner, since there is no easy way to obtain a full qualified URL from a system file.
- If a CDN is used this method will return a URL.
- The asset combiner returns a FQ URL as well, although it didn't always, perhaps this is a relic from that time that was never retrofitted.

This should be flushed as part of the service provider