If a tweet has no `symbols` in its `entities` then `html_for_tweet()`
was failing.
I'm not sure how common this is but, for example, tweets in a downloaded
archive do not have `symbols` for some reason.
The previous change (b366ab5) fixed this, but I'm adding a test
for this case.
I've re-written the parts of `html_for_tweet()` so that it handles
all the replacements of URLs, mentions, symbols and hashtags better.
Mainly to fix#447 but it should be a little more robust generally.
Shamelessly cribbed from https://stackoverflow.com/a/25514650/250962
Passes all tests, but I haven't checked it beyond that.
Fixes#447
