GPX upload improvements

After hopefully having sorted out all problems caused by the rusher migration to a new, faster server I’ve now finally figured out how to improve the user interface for GPX uploads.

The three notable changes are:

• Creating a map now starts with the “Geographic area” tab selected as default instead of “City search”. This is needed to make sure that the map is properly initialized before a possible GPX upload.
• The GPX upload form step near the end of the form workflow has been replaced with a third input mode tab in the first step, labeled “GPX track”. This allows to upload a track, and will then switch back to the “Geographic area” tab to display the selected track as preview
• Uploaded files will now be checked for being valid GPX files