Fix Markdown parsing in project/member guidelines and sign-in/sign-up

Ensure rendered custom Markdown (entered in /admin/application_settings/appearance) looks right in the following places by adding the md CSS class to the relevant containers:

• Project/group member guidelines
• New project guidelines
• Sign-in/sign-up pages

Without the md class, elements like blockquotes and lists are not styled correctly. Closes #440177 (closed).

Screenshots

Given the following input:

# woo

* a
* b

> c! yay

We get the following before and after:

	Before	After
Project members
Group members
New project
Sign in
Sign up