Remove application namespaces

[linc]

Merge before evaluating: #3317

Co-requisite: https://github.com/vanilla/internal/pull/321

Related:

• https://github.com/vanilla/internal/pull/322
• Remove app names from URLs addons#278

This is a prerequisite for refactoring the dispatcher for API v2.

• Remove Vanilla app's SettingsController, refactoring its contents into its hooks file.
• Remove the dashboard/ and vanilla/ URL & menu namespaces.

QA points to consider:

• ResolvedPath's construction has changed, which effects the view counter for discussions.
• Dashboard's category management system was most heavily effected.
• Category API endpoints must continue working as documented. (Also note that /category/get is undocumented but must be tested)
• 'Advanced' page was effected.
• 'Flood Control' page was heavily refactored.
• Many miscellaneous menu navigation items were effected.
• When altering menu & URL items, I left starting/training slashes as-is except where a nearby parallel example contradicted the pattern (e.g. 2 menu items in a row, one with a starting slash and the other without).