Fix `invisible` CSS class

affected, replaced onClick handler with onMouseUp/Down to detect text
selection not trigger onClick handler then

background as well since it's a computationally expensive

now return empty hash instead of throwing a 404 error. When following,
merge into timeline will filter statuses

Textarea: fix clicking on name suggestions

This was broken in c3e9ba6a, which added an onBlur to hide the
suggestions when the textarea loses focus. Unfortunately, this fired
even when the textarea lost focus to its own suggestions box, and that
meant that onSuggestionClick is never called.

Setting a short window in which onSuggestionClick can still happen
before hiding the suggestion area ensures that clicking still works,
while still hiding the suggestions if the user clicks on something else.

Update README.md to match new documentation (missed from previous commit)

Documentation tweaks

used from nginx when Rails fails.

Add a default missing.png header image - fixes #492

Call uniq on the string version of mb_chars tags - fixes #503