Add mouse-over descriptions for module version headings

[brynwhyman]

As a Product Owner, I want to know the definitions of what module versions are available within the CMS, so I can easily understand the support level of modules on my site.

Use case:
A Product Owner can see a module has an installed version of 1.x.x with no "AVAILABLE" version, but there is a 2.x.x "LATEST" version. The Product Owner is unsure on what the "LATEST" version is, or how to get it.

Acceptance Criteria:

• mouse-over text is added for the "INSTALLED", "AVAILABLE", AND "LATEST" headings describing the scope of support.
• mouse-over text is accessible by a screen reader

Notes:

Definitions are available at: https://github.com/bringyourownideas/silverstripe-maintenance/blob/master/docs/en/userguide/index.md#what-do-version-available-latest-mean