Small style fixes
Remove Category and Subcategory models

Category replaced by subclasses, which allows for different fields for each category.
Subcategory will be replaced by tags.

Other changes:
* Submission: Move `filename` field to `File`
* Submission: Rename `user` to `uploader`
* Submission: Fix `voteBalance()` method
* Small Python style fixes
Add LSP1 import script (WIP)
Add login and signup templates