8.x-3.0-alpha3

This iteration adds several new features and greatly expands the unit tests.

Fixes:

• #2629292: An error in 8.x-3.0-alpha2 that prevented custom tags from
  becoming available until the cache was cleared.
• #2629256: Broken [align], [color], [font], [list] and [size] tags fixed,
  because CSS values cannot actually be wrapped in quotes.
• #2629662: Plugins can now be renamed while disabled. This fixes a problem
  that prevented enabling and renaming plugins in the same form
  submission.
• #2629874: Actually block access to the edit/delete forms for uneditable
  tags. The access check had previous just disabled the link.

Features:

• #2629674: All tags, especially uneditable ones, can now be viewed with their
  template code.
• #1754336: A [spoiler] tag has been added to the default tags.