[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
- [x] Create a black list of non standard subfeature type to ignore while exporting them.
- [x] Fix the phase calculation
- [x] Query `Task` and `Submission` to find out all the submission corresponding to a `GFF` given by the user from the database.
- [x] Re-factor code to reduce redundancy.
- [x] Calculate the `end` part for the `##sequence-region` meta-data
- [ ] Name of subfeatures should be generated client side, before the submission is made. Probably in `EditTrack.createTranscript`.
- [ ] Rebase and clean up the history.
Why is this variable called `gff_files`? We are creating only one gff file.