Implement a local/guest account

This PR will be pretty important, as it sets the foundation for multi-account operations. Even if the app technically doesn't support multiple accounts yet. I'll flesh out the details in a bit.

Here's a beginning plan. Since this'll basically be multi-account support, I'm gonna need to be able to make databases that correspond to a specific user. Easiest way to do this is to simply append the database name with the uid. For example, pecunia_0000314128312381.db. I considered updating to an encrypted database but decided to do that in the future. A list of uids where the database exists will also be kept, in the shared preferences.

So I have the saved_users map stored in shared_preferences, but I only meant that to be a record of saved users in the app. If I want to make a local account, I'm gonna need to be able to store the user's email and password hash. It's probably not a good idea to just simply store it in shared_preferences so I'll take a more secure approach.

Initial Account Deeds

[x] #139
[x] Update AuthRepository to store the logged in user after succesfull login

Big Deed, Implement Local Authentication

[x] #141

Continued Deeds

[x] #138
[x] Update logout to first attempt local before remote.

Database Deeds

[x] #145
[x] #154
[x] #148
[x] #153
[x] #149

Enhancements

[x] #136
[x] #137
[x] #146
[ ] #147
[x] #152
[x] #159
[x] #162
[x] #163
[x] #165
[x] #168
[ ] #169
[ ] #171

Bugs

[x] #140
[x] #161
[x] #164
[x] #170
[ ] #175
[ ] #176

nazrinharris / pecunia