Users can lower their rank
The ability to change your rating is limited only on the frontend side, but this restriction also works incorrectly.

<details>
  <summary>GIF</summary>
  
  ![GuHNqeBJgP](https://user-images.githubusercontent.com/43146729/89900849-cbae4680-dbd3-11ea-910f-04e30dd5d312.gif)

</details>

Add a server-side check [here](https://github.com/nim-lang/nimforum/blob/14786d3d4999b1664898ca037c13f39dde0ea138/src/forum.nim#L753).

Something do with [frontend](https://github.com/nim-lang/nimforum/blob/616c6eb1007cacbdbb254ac6eccb07da42613578/src/frontend/profilesettings.nim#L83).

My [main account](https://forum.nim-lang.org/profile/gabbhack) was banned due to this error. I wanted to change my email, I was assigned the rank "EmailUnconfirmed", but I got the message "You can modify anyone's rank...". I tried to assign "Spammer" rank to myself and I succeeded (unfortunately).

I think this is because [currentUser.get().rank](https://github.com/nim-lang/nimforum/blob/616c6eb1007cacbdbb254ac6eccb07da42613578/src/frontend/profilesettings.nim#L83) has changed (to EmailUnconfirmed), and `state.profile.user.rank` has remained the same (Moderated). EmailUnconfirmed > Moderated, so the frontend "allowed" me to change my rank.