Align dialogs and create a BaseDialog class

[kalina559]

This is related to #1356 and #1454.

There's a lot of different dialogs in the app and they're not really aligned with each other - some are completely custom, some inherit from AlertDialog or AppCompatDialogFragment. Sometimes we just call AlertDialog.Builder in an activity.
This makes applying changes to these dialogs more difficult - if we aligned them a bit more we could use a BaseDialog class to implement changes that concern every dialog (like the ones from PRs above).