Deleting completed tasks from DB (#369)

[Aga-C]

What is it?

• Bugfix
• Feature
• Codebase improvement

Description of the changes in your PR

• Added foreign key to tasks table with ON DELETE CASCADE to ensure that removing event will always remove an entry in the tasks table.
  • SQLite doesn't support ALTER TABLE ADD CONSTRAINT, so I had to do a small workaround in the migration.
  • I don't know if I should commit schema JSON. If it's not needed, I can remove it.
• Existing completed tasks without entry in events are deleted during migration.
• Also added code for removing task occurrences to fully support recurring tasks.

Fixes the following issue(s)

• Fixes Data accumulation when deleting a completed task #369

Acknowledgement

• I read the contribution guidelines.

[nofishonfriday]

@Aga-C As the reporter of this issue I wanted to say thanks for fixing it.
I use the Fossify apps on a phone with very limited storage space (S4 mini), that's why I care they behave properly.
I planned to look into this issue myself (currently learning Android dev) but you were so fast, but now I can study your code instead which is also good. :)

Anyway, regarding

I don't know if I should commit schema JSON. If it's not needed, I can remove it.

I recently did a PR for another FOSS app where I had the same question and found this:
https://stackoverflow.com/questions/44322178/room-schema-export-directory-is-not-provided-to-the-annotation-processor-so-we/44645943#44645943

so I commited the JSON file and it was merged.
But @naveensingh will decide finally anway. :)
Have a nice day.