Allow how-to creators to restrict who can see their how-to

[melissachen2000]

Context

In #882, we introduce the new how-to feature, which includes giving gallery curators the option to "expand" access to how-tos. If a gallery A is in "expanded" mode, it means that curators and creators in other galleries curated by the same curator can see the how-tos in gallery A. This might be useful in cases where teachers teach multiple sections or across multiple years and want students to be able to view other classes' how-tos.

We want users to be able to overwrite this setting. That is if the gallery is set to "expanded" mode, creators should have the option to restrict access back to only curators and creators in the gallery that the how-to is in. Back to the example use case, maybe a student only wants their how-to to be visible to their classmates, not to future classes or another class section.

This PR implements this feature as described. We add a "share" panel to the how-to form, where users can change this setting. This panel is always accessible, even if the gallery is not set to expanded mode, since that could be changed later (i.e., users can pre-empt the gallery's access scope being expanded).

Also, small bugfixes:

• The text of the collaboration toggle was not being translated. Fixed by adding a locale string instead of using the TileKind name.
• The text for the collaboration pane talked about being able to add viewers, which currently isn't possible. Fixed by removing that part from the UI texts.

Related issues

• Related Issue Community-created documentation #825
• Closes #

Verification

1. Create user accounts A, B, and C.
2. Create two galleries (gallery 1 and gallery 2) using user A. Add user B to gallery 2. Add user C to gallery 1.
3. Using the user A account, add two how-tos to gallery 1. Set the access scope for one of the how-tos to "restricted."
4. Set gallery 1's scope to "expanded." Access gallery 1 using user B. User B should only see one how-to.
5. Access gallery 1 using user C. User C should be able to see both how-tos.

Checklist