Closes #399. Adds a setting (under "General Settings") to select a
custom directory to store new apps in, replacing the default `dyad-apps`
folder.

In order to make sure that users don't lose access to older apps, I
opted for creating symlinks inside the new folder to the old app
locations. I went with symlinking because moving every single app could
be an expensive operation depending on how many apps there are, and the
user might not want that. However, this is inconsistent with the import
apps and move folder features (i.e. #2000), which copy the apps instead
of symlinking.

I'm happy to change the approach on request. Some options I've been
thinking about:
- Add a button in the settings which moves all the apps (replacing the
symlinks) after the user chooses a new custom directory. This way, the
user would get to choose.
- Convert all of previously-created apps to absolute paths, which avoids
all of the symlinking. The only potential issue with this is that if the
user wants to move the apps to their new directory after all, they'd
have to use the "move app" feature for every single app, otherwise the
database wouldn't get updated. As is, they at least could just delete
all the symlinks and mass-move all of their apps into the new directory
(though they'd currently have to do this outside of Dyad).

Also, since I'm tentatively adding in the use of symlinks, I modified
the move/copy/delete app features so that they always target the true
location of the app, not the symlink.

I can also write tests if desired.
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