[WIP] export submissions to GFF3 format

This is a Work in Progress[WIP].

- [x] Change indentation to 2 spaces.
- [x] Verify the generated GFF3 file using http://genometools.org/cgi-bin/gff3validator.cgi
- [x] Handle more than one submission
- [x] Handle more than one transcript in a single submission
- [ ] Write all the submissions on the same genome in a single file. The genome is given by the user in the form of the `.gff` file in the database.
- [ ] Name of subfeatures should be generated client side, before the submission is made. Probably in `EditTrack.createTranscript`.
- [ ] Rebase and clean up the history.
Iterate over submissions per gff file instead. Instead of `Submission.each`, do `submissions.each`, where `submissions` returns all submissions per gff file.