mutation 'changeMissingStatus' for Phase 2 Store Refactor

[jarmoza]

This PR closes #275 by implementing the following:

1. A mutation changeMissingStatus that takes a column name, value, and a boolean flag indicating whether or not the value should be marked as missing or its missing status should be removed.
2. A unit test for changeMissingStatus in store-mutation-changeMissingStatus.cy.js that tests adding and removing the missing status of a value.
3. Previous uses of the functions designateAsMissing and declareAsNotMissing in annot-categorical and annot-missing-values have been converted into using the more general changeMissingStatus mutation.
4. An unused reference to designateAsMissing in the test for annot-continuous-values (ostensibly for a future 'mark as missing' feature for this component) has been amended for the new mutation name and parameters.

[surchs]

Nice! Looks good to me @jarmoza 🎉

[rmanaem]

Nicely done!
I left two comments, one for adding a comment as a reminder I leave up to you and the other for whether missingValues should exist in store state.