Close #1832
Part of #1808

This

• Changes to fetch politeia proposals after wallet startup without having to enter the governance page
• Adds two new config entries to store the latest timestamp and block height when politeia data was successfully fetched
• Uses those two config items to determine whether a vetted or active vote proposal are new ones (either newly vetted or newly voting)
• Shows the appropriate notification ui

There's an outstanding issue due to decred/politeia#618: Given we only have the timestamp for proposal creation (vs proposal vetting for discussion) this version has a bug in that proposals that have been vetted after the user has fetched the latest batch of proposals will not be marked as modifiedSinceLastAccess (because the user has already passed through the block height of the proposal creation).

Alternatives for this are:

• Wait for Return vet timestamp on /vetted politeia#618 to be solved, and than change which timestamp is used to determine new proposals (preferred method)
• Locally store the proposals that have been seen and check after loading the proposal list whether this has been seen