Remove username case-sensitivity requirement when adding committer to plugin

[raamdev]

When adding a committer to a plugin on the plugin Admin screen, using the improper username case (e.g., kristineds instead of KristineDS) and clicking "Add Committer" results in the page simply refreshing with ?error=no-user appended to the URL.

​Otto wrote:

we should just try to find those places where case matters and make it not matter. That should be fixable on that admin screen

[Otto42]

After testing, it appears this field already ignores case for usernames. I was able to add my test user named "Notto" using "NOTTO", "nOtTo" and "notto" with no issues.

Upon attempting to add "kristineds" I did encounter a problem. Seems to be a caching issue. I've adjusted the plugin add-committer code to correct the cache issue.

Commit in dotorg:10709.

[netweb]

Related: #588