[Feature] Admin users page improvements

[HebaruSan]

Motivation

The admin pages include a users table, which has a few deficiencies:

• Horizontal scrolling is required if the values on the current page are too wide
• The full description is shown, some of which are very long
• The forum, IRC, Twitter, and Reddit fields are shown, but the KerbalX and GitHub fields from #381 are not
• We're still getting a high volume of fake/spam users ("Pay for Research Paper Writing Services Are you wondering if it would be possible to buy research paper writing services from online sources? ..."), and there's no way to manage them

Suggestions

• Enforce a maximum cell size and put the full text into the tooltip
• Add columns for KerbalX and GitHub
• Add a button to delete users (see #316 for general user deletion logic)