Allow installing older version of an extension if the latest version is
not compatible
- Disable Update button when latest update available isn't compatible with
Brackets (previously was erroneously left enabled in Installed tab).
- Clarify in _renderItem() when button is disabled (making it more obvious
that its tooltip string is always correct).
- Make it more clear that flags like 'context.isCompatible' are specifically
referring to the latest version available, not the currently installed
version.
Merge branch 'pflynn/strings-latest-compatible-ext' into pflynn/latest-compatible-ext

Conflicts:
	src/nls/root/strings.js
- Update Extension Manager UI to use new warning string
- Add more unit tests for Extension Manager install/update button state &
ExtensionManager.getCompatibilityInfo() utility
- Fix crash when calling getCompatibilityInfo() directly on an installInfo
(instead of registryInfo) - though I think this now only happens in unit
tests
- Ensure getCompatibilityInfo() never returns isLatestVersion unless
isCompatabile is true
Merge pull request #5653 from adobe/pflynn/latest-compatible-ext

Allow installing older version of extension if latest not compatible