(Do not) keep the original GPX file

From a "purist"/"archival" perspective, it seems desirable to have Fietsboek keep the original GPX file that was uploaded.

However, given #13 (closed) and #18 (closed), it is clear that there is also desirable functionality that would break this promise. Thinking further, we might also want to support other formats than GPX in the future or store tracks in a more efficient binary format internally.

While we could always fall back to storing the original file, this seems a bit unnecessary - why would it be Fietsboek's job to do that? Where could the user access this original file, what would it be used for? Instead, I would propose the following changes:

• We make it clear that Fietsboek is not for archival purposes, but rather a library/browser/sharing site - this is also represented by the fact that Fietsboek is a web service and does not organize your local GPX files.
• Fietsboek does not store a byte-for-byte copy of the original file, but rather stores a equivalent representation (like MyTourbook does). For the beginning, this will be a (slightly modified) version of the uploaded GPX - but that might change in the future.
• We do our best to keep the properties of the original file, but we make no guarantees about the internal storage format - that also means no guarantees about embedded GPX extensions or other metadata, except for the information Fietsboek processes.
• People that want to archive their GPX files should do it with a tool better suited for it, such as a properly backed up local folder, or cloud storage.