Add ability to hide forms trash

[matthew-white]

@yanokwa brought up the idea of hiding the forms trash so that the page appears less messy. From Slack:

One solution could be adding a ▶️ so you can toggle it open and closed. Should probably default to closed since we can’t easily persist views?

I've filed #689 about saving user preferences. Whether the trash is shown or not could be one of those preferences.

[matthew-white]

@getodk/testers, this issue is now ready to be verified. It is available on the staging server. There is a new button at the top of the form trash section to show or hide the trash. The trash is shown by default. The choice/preference to show or hide the trash is saved on the backend. For example, if you hide the trash, then you refresh the page or open the page in a new tab, you should see that the trash is still hidden. Hiding the trash in one project will not hide the trash in another project: the choice to hide the trash is limited to the individual project.

[matthew-white]

The choice/preference to show or hide the trash is also limited to the user. For example, if I hide the form trash in a project, then another user visits the same project, the other user should see that the trash is visible, not hidden.

[srujner]

Tested with Success!

@matthew-white Is there a possibility to add something similar for the Archived Projects section?

[matthew-white]

Ah that's a good idea! I've added it to #719 so we don't lose track of it.