Fix incorrect href with subdir

[rugk]

Fixes #6

[rugk]

My second commit now fixes another issue:
With the first version both admin plugins I mentioned now failed to print the URL correctly. (they added another path to /customEtherpadSubDir/customAdminDir so it is /customEtherpadSubDir/customAdminDir/admin.

Now it works, but it does not have to work in every Etherpad installation. This is probably why the other plugins have prefix handling.

[Gared]

If you're on the "Troubleshooting information" page, the link to "Message to all" is broken.
After I had a deeper look into the code this is a problem how etherpad build urls and should be fixed there for all plugins.

[rugk]

So have you found/opened an issue in the Etherpad repo?

[Gared]

@rugk Not yet. You can create an issue if you want

[rugk]

Well... I think you can describe the issue better as you dug into the code. 😄

[Gared]

Just created a ticket: ether/etherpad-lite#3001