New navigation pattern for journals page

[joemull]

Fixes #358.
Fixes #400.

This PR reconsiders the Journals page navigation in light of #358 and #400.

• Remove the filter. Rationale: making it accessible is too complex for the benefit, and I have questions about whether people event want this feature in the first place. It is a legacy from the other OLH themes. I'd like to collect feedback from OLH editorial on potentially leaving it out, as a part of this review process.
• Create a gallery of journal covers at the top of the page, to serve as a quick navigation.
• Create a back-to-top link at the bottom of the viewport, following best practices.
• Fix the hierarchy issue between h2 and h3.

[StephDriver]

I think this is going to the Design Meeting @joemull ?

Initial thoughts

• I agree that this is an improvement over focusing on a more accessible filter.
• this makes a beautiful showcase - and better journal discoverability.
• for a gallery, I do think the covers all need to have the same dimensions, or be on cards that have the same dimensions, the way they are different sizes looks messy.
• I think whether one wants a gallery or list is personal preference, and I think some people will be very certain abotu which they would prefer. For example, I prefer my ebooks as a list, but I think a lot of people like them as cover images. So I wonder if we could have a toggle above the gallery for "cover : text" so that people can either view the gallery by cover image, or they could view a text list. A bit like on a computer file system or book libraries like Kindle where you can have it by book cover, or a list of books.