Move subscriptions to database - Phase 2 (#107)

* Added custom account passing

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Context already given in class constructor

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Migrated to new database

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Added tests

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Added `AppDatabase.setInstance`

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Added work testing

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Removed async calls

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Updated `getAccount`

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Added testing account updater

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Changed order by naming

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Removed all account-related things

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Added copyright notice

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Refactor

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Extended KTDoc

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Removed `InitCalendarProviderRule`

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* [WIP] Rewrite migration to DB

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Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>
Co-authored-by: Ricki Hirner <hirner@bitfire.at>