Add notice that account deletion is permanent

[pixeldesu]

Too many people think that account deletion on Retrospring follows the same scheme of services that either never delete your data, or delete it only after a month.

Currently account deletion is a red button with an additional confirmation, looks like we'll have to add more disclaimer text (and more red) to that section.

[raccube]

This was partly done in #591

[pixeldesu]

I think this in fact is enough, if people don't read a warning that says the action is IRREVERSIBLE (in that casing), then a box with red text isn't going to be read any more than that.