feat: add placeholders for social network profile input (resolves #100)

[greatislander]

• [x] I've read the guidelines for Contributing to the Platform Co-op Toolkit
• [x] This isn't a duplicate of an existing pull request

Description

Adds placeholders for social network fields on individual and organizational profiles, and derives labels from SocialNetwork model rather than requiring a custom filter or initial data.

Steps to test

1. Add or edit an individual profile.
2. Add or edit an organizational profile.

Expected behavior: Social media fields include placeholder text which shows the required format of the input.

Additional information

Not applicable.

Related issues

Resolves #100.