Fix-RTE was not initialised when it is mounted 2nd time

[vicky-primathon]

Description

Remove RTE instance from tinymce editor instances when component is unmounted. Tinymce internally keeps a list of editors initialized. And we need to remove editor from that list when component is unmounted.

Fixes #785

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Manually tested that RTE is initialised every time a page is loaded.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[Nikhil-Nandagopal]

@vicky-primathon I see some tests failing

[riodeuno]

Duplicate of #690. @Nikhil-Nandagopal Please close the one you find inappropriate.