Show the feed/folder title over the entries list even while loading entries.

There is no reason to hide it.
Showing the start page now only depends on whether there is a currently selected feed or folder.
Undo commit e01c772a8574b3985e2ba5e343a41a4ee7ff9d54

With the changes in that commit, the start page was displayed over the "loading..." message while adding a new subscription.
Buttons in the navbar are not disabling while loading entries.

It is a bit disconcerting for the user that buttons briefly disappear when he clicks on a feed.
Remove the loading_entries boolean flag, which indicated that the first page of entries was being loaded.

This is not necessary and it is redundant with the loading_entries_busy flag, which indicates that an entries page is being loaded. If we need to check if the first page is being loaded, it is enough to check the loading_entries_busy flag (which would indicate that entries are being loaded) and the entries_page root scope variable, which would tell us if the page being loaded is actually the first one.

Therefore the loading_entries flag is totally redundant and has been removed.