Error when generic OAuth provider doesn't return a user ID

Fix trailing backslash on lists in Classic editor

Fix SQLite database lock on Reader

Previously, when editing a post with an unordered list in it via the
Classic editor, backslashes (\) would get added to the end of each list
item. This fixes that.

Closes #480

Fix "Collection.hostName is empty!" log on Stats page

Don't render title as list item

Don't include img alt text in post Summary()

Remove additional slash in /feed/ redirect

Fixes #468

Fetching posts for the Reader involves an additional query on each row,
which previously ran into our connection limit to the database and caused
it to lock up. This increases the connection limit from 1 to 2, to allow
this.

This is meant to be a quick, safe fix, but there could always be a better
solution.

Fixes #467

Fixes #306

...on the OAuth access token inspection call. This returns an error and
privately (via logs) prompts the admin to add a `map_user_id` config value.

Fixes #469

This fixes an issue where "12. April" would get rendered as "1. April" because
it looks like a Markdown list item to our renderer. Now, we parse titles as
titles, instead of standalone text, which causes the renderer to give us the
results we want. This also adds some basic tests for the applyBasicMarkdown()
func.

Closes #470

Web Monetization fixes + exclusive content

Ref T770

Final API changes

This forces xgo to use Go 1.15, to work around a bug with Go modules:
https://github.com/techknowlogick/xgo/issues/109#issuecomment-835494720

This also uses the correct Darwin and Windows binary names to prevent
failures in the `make release` process.

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Draft list paging

Minor WYSIWYG and miscellaneous fixes

This fixes logging in when developing on newer versions of Chrome.