Help prevent users from losing annotations by not saving

**Describe the bug**
Users don't always know what "Post" means. They are used to auto save features, and sometimes think posting will be permanent as opposed to an auto-saved draft. This means they sometimes close a page with many open annotation cards, losing everything.

Similarly, sometimes users will miss the error message telling them an annotation hasn't saved after clicking post (for example, if they are offline when trying to post) and believe they have successfully saved an annotation.

**Expected behavior**
Some solutions could be:
1. Change the "Post" button to say "Save"
2. Give users a pop up when they try to leave a page with unsaved annotations