`txindex` related changes:

- Fetch block transactions with `getrawtransaction <txid> <blockhash>`. This works without `txindex`.

- Tolerate missing previous output information and render transactions without it.

Pruning related changes:

- Fallback to `getblockheader` instead of `getblock` for pruned blocks. The txid list and weight/size information will be missing.

- Tolerate missing block stats.

…txindex is available

i.e. `/tx/<txid>?height=<height>`. This can work without txindex.

When txindex is disabled, the block page links to the transactions with
this parameter set.

…mempool

But still avoid unnecessarily loading prevouts

Also:
- Use nicer-looking /tx/<txid>@<height> URLs
- Simplify by removing some unnecessary code
- Fix a bug where searching for invalid block height hanged

Also changed session.userMessage to render as markdown,
hopefully that's okay?

Using the information from `getmempooletry`.

This only works with Electrs and requires enabling BTCEXP_ELECTRUM_TXINDEX.

See: romanz/electrs@a0a3d4f