Linkstate refactor (#609)

* Linkingstate Refactor, and displaying link info in rightpanel

Big refactor to LinkingState
    Collects descriptive/user-facing labels into FilterState
    Unifies/cleans up some logic
    Adds LinkTypeDescription, a string enum which can be used
    to easily switch/case between various cases of linking, and
    codifies the logic in one place (currently only used for linkInfo)

Adds Link info to creator panel, near SelectBy dropdown

Bugfix: Disables linking from Attachment columns
Bugfix/Behavior change: changed linking with empty RefLists to better
match behavior of refs.
    for context: Linking by a blank Ref filters to show records with a
    blank value for that Ref. Previously this didn't work with RefLists. 
    Linking from a blank refList would show no records 
    (except in some cases involving summary tables)
    Fixed this so that linking by a blank val consistently means "show
    all records where the corresponding col is blank"

app/client/models/entities/ViewSectionRec.ts

@@ -1,5 +1,5 @@
 import BaseView from 'app/client/components/BaseView';
-import {LinkingState} from 'app/client/components/LinkingState';
+import {EmptyFilterColValues, LinkingState} from 'app/client/components/LinkingState';
 import {KoArray} from 'app/client/lib/koArray';
 import {ColumnToMapImpl} from 'app/client/models/ColumnToMap';
 import {
@@ -637,7 +637,7 @@ export function createViewSectionRec(this: ViewSectionRec, docModel: DocModel):
   }));
 
   this.linkingFilter = this.autoDispose(ko.pureComputed(() => {
-    return this.linkingState()?.filterColValues?.() || {filters: {}, operations: {}};
+    return this.linkingState()?.filterColValues?.() || EmptyFilterColValues;
   }));
 
   // If the view instance for this section is instantiated, it will be accessible here.