Add capabilities to control the Kaltura button in HTML editors

[estevebadia]

This PR adds capabilities to TinyMCE and Atto plugins so that the Kaltura button is shown only for those users with required capability. By default, the capability is enabled in all role archetypes from student to manager so there shouldn't be any backwards compatibility issues.

Concretely:

/lib/editor/tinymce/plugins/kalturamedia

• Add the tinymce/kalturamedia:insert capability. The kaltura button is shown only for users with that capability.

/lib/editor/atto/plugins/kalturamedia

• Add the atto/kalturamedia:insert capability. The kaltura button is shown only for users with that capability.

Note that this PR implements the same feature than #356 but this one also adds the symmetric feature for tinymce plugin.

Are you interested in this feature?

Note: I'm contributing to this repo for the first time, please guide me on the way to proceed!

[kaltura-hooks]

Hi @estevebadia,
Thank you for contributing this pull request!
Please sign the Kaltura CLA so we can review and merge your contribution.
Learn more at http://bit.ly/KalturaContrib

[estevebadia]

Hi, Just signed the CLA