Remove trailing slashes and `.git` from repo URLs at fetch time

[pgengler]

When fetching addon data, repo URLs should have trailing slashes and trailing .git stripped before they're saved to the DB. This will eliminate the need to do this on the client side (like in emberobserver/client#115) and will generally look better in the UI.

[pgengler]

This should also the point for any other repo URL normalization. For example, replacing git:// urls (as with https://emberobserver.com/addons/ember-material-addon) with https:// ones as appropriate.