Implement missing values component

[surchs]

This PR implements the version of the missing value component we have discussed in #58. The component does not detect or create missing values, it only does two things:

• for the currently relevant column names, show the missing values currently kept in the global store
• provide a UI element (button) to declare any of these values as "not missing". this will trigger an event chain and update the global store by removing this value from the array of missing values of its columnName

Writing to missingColumnValues in the store via the setMissingColumnValues mutation was not reactive, because Vue doesn't track indexed changes to Objects (see https://v2.vuejs.org/v2/guide/reactivity.html). I have replaced the store logic with an Object.assign statement. This will continue to work with the partial updates we are receiving from the annotation tab,

Example:

let state_missingColumnValues = {
        "sex": ["na", "999"],
        "diagnosis": ["AD", "none", "dunno"] // AD is not a missing value, so we remove it
};
const new_missing_values = {"diagnosis": ["none", "dunno"]};
state_missingColumnValues = Object.assign({}, state_missingColumnValues, new_missing_values);
console.log(state_missingColumnValues);

// { sex: [ 'na', '999' ], diagnosis: [ 'none', 'dunno' ] }

Another thing I did here is to give the missing Value Component direct access to the store state. This is different from what we do for the other components and what was previously done, where we give only the pages (e.g. annotation) access to the store and then provide/inject or prop-chain the information down to the components. Two reasons why I did it this way here:

• the component already has state access for the getter (valueDescription)
• none of the other components needs access to this information so provide / inject seems a bit overkill. similarly so would be props edit: I just realized that this is not true. Other components will need this as well. So yeah, maybe we should use provide / inject
• it is easier to see what's going on while inside the component (this is a thing from the state).

But open to change to props or provide / inject.

Sidenotes:

• I'm not sure why I'm still getting white-space diffs here, I thought I had solved that by running eslint beforehand. Sorry about that.
• because this gets implemented before we have a way to "create" missing values, and because the missing values component no longer makes its own guesses, I stuck some temporary event emitter in the annotation page to create some missing values to debug with (targeted to our example.tsv file). I'll remove these once we have a way to add missing values!

Resolves #58

[jarmoza]

Some minor formatting changes. We have to check why our linter has not picked up on missing semicolons.

Feel free to merge after you fix the formatting issues.