Add seed UX

[lwrubel]

Changes recommended by Cory (for seed_create page):

• Twitter user timeline seeds:
  • Change “screen name” to “username”.
  • Help text: “Handle or screen name for the account. For example: @Potus or @nytimes. If the Twitter user changes this name later, Social Feed Manager will update to use the new name.”
  • Explanation for User ID: “Twitter’s internal numeric identifier for the account, does not change over time, although the username may. For example: [provide a sample UID for a public account].”

Further, for Tumblr seed_create:

• Update help text to: "Provide either the standard blog hostname or URL, e.g. nasa or nasa.tumblr.com. More info on blog hostnames."