Issue Bundle

Adds the `TickitIssueBundle`

Must not be merged before #133.

TODO:
- [x] Add forms for `Issue` and `IssueAttachment` (see #133)
- [x] Add issue listing + filtering
- [x] Add basic issue create / edit (see #136)
- [x] Add issue delete (no need to worry about soft-delete at the moment)
- [x] Decide on storage strategy for issue numbers w/ @markwilson 

At a later date we will need to add the additional functionality available from the Issue overview screen (commenting, changing assignee/status/priority etc.)