[FEATURE] Starring system for linked vocab, stories

[saxoncameron]

I propose a system where users can star linked vocab and stories:

1. Linked vocab that is starred would appear on the front of the card, taking precedence over the default words determined by frequency.
2. Stories that have been starred would be "pinned" to the top of the story section, and highlighted/visibly-starred to be distinguished from the other stories.

Pinning stories would be espeically useful to make it clear which mnemonic story is the "chosen" one, because users will typically read through the list of provided stories, pick one, and then commit that single mnemonic to memory. In future reviews, finding that same chosen mnemonic among the others is sometimes tedious.

[saxoncameron]

I envision this as an on-hover icon UI, perhaps as one of three actions, including:

• editing (#160)
• starring, to "pin" an example word to the card front (#114)
• deleting/ignoring (#158)