Fix Area to GlobalArea Flip Flop

Fix Area to GlobalArea flip flop

If an Area is changed into a GlobalArea in a theme file, or if a new
theme is installed which has an GlobalArea with the same name as an Area
in the previous theme, the view will alternate display of the Area and
GlobalArea after every other page load.

The is true if the reverse is done and a GlobalArea is changed into an
Area of the same Name

- Add `$arIsGlobal = null` parameter to `Area::get()`
- Store seperate cache for when 1 is passed into `$arIsGlobal` to
  seperate caches
- Pass in `arIsGlobal` table field when rescanning area permissions to
  avoid filling the cache with the wrong one
Update `Area::get()` docs

Update `Area::get()` docs
Merge pull request #1246 from mkly/fix_area_global_flip_flop

Fix area global flip flop