Join separated client and server page components

[brenoferreira]

Because of legacy issues, there are three pages that have separate server and client versions:

• BrowsePage
• CollectionsBrowsePage
• PhotoDetailPage

The reason for that is that for some time we used a JS library to render the image grid. This library used browser specific APIs to render the image grid. Therefore it didn't work on the server renderer. So we had to split those pages into two different versions, one that didn't render the image grid (for the server) and another that did render it (for the client).

But in #538 we dropped the usage of Masonry Layout in favor of a CSS-only implementation of the image grid. This should make it possible to join those pages that are split into two different client and server versions. It should also make it possible to have a single router file.

It should be possible to:

• delete the server versions of the page components
• bring the code of the mixins back into the client specific page components (with the exception of ServerPrefetchProvidersMixin which is used for different purposes)
• delete the server router and have a single router file (caveats: check the behaviour of scrollBehavior in the client router on the server.