Strip HTML from <title> attribute; fixes the <br /> that is currently showing up there

[shinypb]

What is changed?

Currently, the web site at tikv.org has a literal <br /> in the <title> tag:

This changes the template to run the description through Hugo's plainify function, which strips out all HTML tags and returns a plain text version of the input.

Tada:

Any related PRs or issues?

N/A

Which version does your change affect?

N/A