Phase 2 Refactor: Missing values functionality + integration with Annotation page

[jarmoza]

This implements #381 along with a few adjustments for the missing values refactor. There is also some cleanup - removal of now-extraneous functions, removal of old comments, and updates to relevant unit tests

annot-continuous-values.vue

• Restores missing value functionality (mark and unmark) to the annot-continuous-values component

annot-missing-values.cy.js

• Removed an old mock getter for the value description (now called getValueDescription)

store-getter-getMissingValues.cy.js

• Update to the unit tests given new way missingValues key in the data dictionary is initialized (see index.js for more info)

store-mutation-initializeDataDictionary.cy.js

• Update to the unit tests given new initialization for the data dictionary with missing values

annotation.vue

• Removing no longer necessary imports
• Removing commented out code

index.js

• missingValues for columns in dataDictionary.annotated are now initialized inside initializeDataDictionary. The reasoning behind this is that this results in cleaner code down the line where we don't have to continually check for the missingValues list's existence
• When a column is linked or unlinked to a category, columns in dataDictionary.annotated are now reverted to their default/userProvided values + new empty missingValues list

[jarmoza]

@rmanaem Changes pushed.

[rmanaem]

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