Ui Improvement: Collect user discord tag

[DonNicoJs]

Currently we do not have a field to collect the user DiscordTag.
Since the vue community hangs out mostly on Discord (https://vue-land.js.org/ )
It would make sense to add this field to the user profile.

With this issue we should achieve:

• Modifycomponents/UserProfileForm.vue to collect the discordTag, right after the twitter profile
• Modify components/PersonDetails.vue to display the discord icon (and link to discord profile ) after twitter icon on the top
• Modify django/people/models.py to include discordTag as a field
• Verify that everything works is and properly wired.

[lloydjatkinson]

Would be useful to include their role tag as well - e.g. Core members, Moderators, MVP, all the rest.