Migrate away from OwnCloudClient to NextcloudClient [1]

[ZetaTom]

This issue serves to outline and track the changes made to migrate from the legacy OwnCloudClient to the current NextcloudClient.

As this endeavour requires a lot of mostly minor changes it has been decided that it shall be split into several "smaller" pull requests.

Part of this first set of changes are the following:

• #1271
  • helper classes to ease the transition and refactoring
  • general changes
• #1272
  • https://github.com/nextcloud/android/pull/12266
  • changes related to notifications
• #1273
  • https://github.com/nextcloud/android/pull/12267
  • changes related to direct editing
• #1294
  • https://github.com/nextcloud/android/pull/13178
  • changes related to sharing
  • server status and info operations
• #1295
  • https://github.com/nextcloud/android/pull/12464
  • changes related to comments