Combine EditPhrasesKeyboardFragment and AddPhraseKeyboardFragment

These fragments are both subclasses of EditKeyboardFragment, and have very similar functionality. EditPhrasesKeyboardFragment should be able to handle both jobs, with the following changes:

• Add a boolean nav arg that identifies whether a phrase is being added (a var already exists in the fragment for this, but its value is always false)
• Pass the appropriate value for said nav arg when editing vs. adding a new phrase (tapping existing phrase or using + button)
• Alter nav graph so that the + button on EditCategoryPhrasesFragment opens an Edit fragment instead of an Add fragment
• Add the function addPhrase to EditPhrasesViewModel: see AddPhraseViewModel
• Probably replace use of deprecated ILegacyCategoriesAndPhrasesRepository in PhrasesUseCase (may be best broken out into future work)