Lazy draw dropdowns to improve performance

[dsevillamartin]

Fixes #876

Changes proposed in this pull request:

• Add lazyDraw attribute to Dropdown component
• Make PermissionGrid dropdowns use lazy draw

Reviewers should focus on:

• Do we like this implementation? Did we have something else in mind?
• Could this potentially be breaking if an extension has logic that occurs on the items() call of a Dropdown?

Confirmed

• Frontend changes: tested on a local Flarum installation.

[davwheat]

I assume the comment about breaking exts was meant for getMenu?

I think we should take that risk. getMenu was called whenever view was called, and I can't see it being done anywhere else. If for some crazy reason someone extended getMenu with anything not related to just getting the menu, they should be able to move that logic to view instead.