Change the "look for an instance" button to explore

What does this MR do?

Changes the essentially useless Look for an instance button to an Explore button which goes to /explore

This button does also hide itself if local timeline view is disabled for guests like all the timeline widgets do on the homepage.

Closes #872 (closed)

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added enhancement label

requested review from @dakkar and @fEmber

changed title from Change the "look for another instance" button to explore to Change the "look for an instance" button to explore

approved this merge request

approved this merge request

mentioned in commit 3de47474

merged