Fixes: #281

In a project with ~300 checks,

* HTML size (uncompressed) before: 772KiB
* HTML size (uncompressed) after: 703KiB

cc: #547

Fixes: #354

When an account has a password, replace "Set Password"
button's label with "Change Password"

The line height in the ping request body is being set by the bootstrap.css body tag to 1.42857143. The additional line height makes request bodies with unicode characters have spacing between lines that shouldn't be there.

Problem: if you use uBlock Origin, and enable the
"Fanboy's Social" filter list, Healthchecks does not show
Telegram or WhatsApp icons. This is because the filter list
contains "##.icon-telegram" and "##.icon-whatsapp" entries.

This commit changes the CSS class prefix to "ic-". So we're
now using icon classes like "ic-telegram" and "ic-whatsapp".

As a bonus, we save 2 bytes in HTML per displayed icon :-)

Fixes: #428