Allow users to navigate back to Artemis from external webcontent

[ungaralex]

This was the result of a discussion we had way back, where we decided to not let users break out of the "Artemis space" since we can't really guarantee what will happen as only the usage with the Artemis website is tested.
Let's also say a teaching assistant links a code example and the student clicks on it. He is now outside of our domain and has no obvious way of going back, apart from re-opening the IDE/reloading in the settings/left-click context menu.

Now, for this specific case, we absolutely should allow users to use external login mechanisms. What I wanted to have and would be my favorite solution, is to remove the pop-up completely and then have a button above the browser (so an IDE-native button on top off the tool window next to the default ones), which always brings you back to the Artemis homepage if you have any problems whatsoever. An alternative would be to have a prominent button overlayed, having the same functionality, as soon as we detect a user leaving the domain.
No matter what, removing the current blocking of external domains should be the first step @nutatuna .

Originally posted by @ungaralex in #30 (comment)