instead of linking to posts only on a user's blog.

Ref T681

This moves `hostName` to the `Collection` struct, where it's needed. The
field is populated after successful `GetCollection...()` calls.

This isn't the cleanest way to do things, but it accomplishes the goal.
Eventually, we should accept the AppCfg to `GetCollection...()` calls,
or make them `App` methods, instead of `datastore` methods.

Ref T613

Previously, our hashtag parser would indiscriminately replace
hashtag-like text with hashtag HTML -- including in places it shouldn't
have, like inside code blocks. Along with the v1.7.0 changes to
writeas/saturday, this fixes that and closes #6.

As a bonus, strings of #spaceless#hashtags#in#a#row are now rendered
correctly.

Previously the logic was mixed up -- multi-user blogs would always load
collection number 1.