MDL-75331 editor_tiny: Limit formatting options

This commit removes several contentious formatting options for now,
pending further consideration, in particular it removes:
- forecolor
- backcolor
- fontfamily
- styles

The two color plugins are written in such a way that the chosen colour
is embedded into a style tag in the generated content. This is not
ideal as it removes these decisions away from theme developers and upon
theme changes to a site, is likely to lead to inaccessible and/or broken
content.

We feel that a new colour plugin will be required which links to known
colour style classes within the theme to allow a subset of colours which
are guaranteed to work with the theme.

The fontfamily plugin is removed pending further administrator
configuration in a future release.

In the case of the 'styles' plugin, this duplicates large parts of the
Format menu into which it is placed but does so in a way that does
respect the configuration of other areas - for example the block_formats
tag is ignored in this menu.

Part of MDL-75966