Add RichText field type

[littleforest]

This adds a RichText field type, completing the work started in #1660.

[littleforest]

Will look into the failing tests, some of them are not failing for me locally :(

[pablobm]

I had a look. It appears to be order-dependent.

Specifically, when I run en example from spec/generators/routes_generator_spec.rb first, followed by spec/administrate/views/fields/rich_text/_form_spec.rb, the latter fails. Looking into it a bit more, I have narrowed it down to the reset_routes call in the after hook, and in turn to the Rails.application.routes.clear! within it:

administrate/spec/support/generator_spec_helpers.rb

Lines 38 to 41 in 852b62c

	def reset_routes
	Rails.application.routes.clear!
	load "spec/example_app/config/routes.rb"
	end

So it looks that, whatever happens there, it deletes the rails_direct_upload_url helper, making the example fail.

Thoughts?

[santosh-1987]

Are we planning to release this anytime soon - Need this badly for actiontext

[nickcharlton]

@littleforest, are you able to pick up from @seanpdoyle's comments here?

(If not, I can do so!)

[littleforest]

@nickcharlton I will see if I can find time to work on this second week of January. Might go faster if I can find time to pair with you or @seanpdoyle .

[nickcharlton]

@littleforest, sorry, only getting back to this one now. How did you get on? Feel free to drop some time on my calendar if you've like to pair on this.

[seanpdoyle]

@littleforest the asset changes merged as part of #2397 have introduced merge conflicts.

If you're interested, I'm willing to give resolving those conflicts a try and force-pushing to this branch.

If not, and you'd rather handle that yourself, that's great too!

[littleforest]

I'm willing to give resolving those conflicts a try and force-pushing to this branch

That would be amazing! Thank you.

[littleforest]

@seanpdoyle @nickcharlton @pablobm this is ready for re-review.