Better public hashtag pages

[lynlynlynx]

Change background color style of hashtag description

It's because

1. Background color setting by its height breaks with a long tag or in some languages. (it is difficult to cover all cases by wide margin between hashtag description and features)
2. In order to display all four features of mastodon.
3. The timeline on two background colors makes me feel inconsistent (the timeline on about pages is only on single background color)

The problem of 1. reason looks like

​

This is how it looks like, how do you feel?

You can also check this on my instance https://md.cryo.jp/tags/test .
(Mobile view is not changed at all.)

Fix overflowing timelines in hashtag and about pages.

Before and after images

This is related to #5237 .