Signup popover owns url

[ChristopherChudzicki]

What are the relevant tickets?

Followup to #1565

Description (What does it do?)

This PR has no functionality change, but makes some rebasing / conflicts easier on the NextJS branch. This PR tweaks two changes in introduced in #1565. Two changes introduced there were:

1. moving SignupPopover to ol-components
   • this had the undesirable consequence that SignupPopover no longer owns the signup url, which seems appropriate. Additionally, many more components are accessing useLocation, which caused some conflicts on the nextjs branch.
2. introducing a MUI_DRAWER_Z_INDEX constant.
   • this is more simply accessed via theme.zIndex.drawer

Change (1) was made so that LearningResourceExpanded could use the signup popover . This PR moves that logic to the LearningResourceDrawer`, which is more similar to how we handle the signup popover for learning resource cards and is also how we handle the "add to list dialogs".

How can this be tested?

1. As an unauthenticated user, check that the signup popover opens when...
   • user clicks bookmark button on cards OR in resource drawer
   • user clicks subscription button on channel pages
2. Check that the add to list dialogs still open appropriately.

[abeglova]

👍