US #1212: Show info about maximun size allowed for the avatar and attachments

NOTE 1: @Xaviju I've made a 'push -f'
NOTE 2: You have to add the max size allowed (in b) to config/main.json:
```
    "maxUploadFileSize": 7340032
```
NOTE 3: When you merge this PR you must to update the taiga-doc section about front configuration (https://github.com/taigaio/taiga-doc/blob/master/setup-production.adoc at 4.2)