(core) Implement UI for trigger formulas.

Summary:
- Implement UI with "Apply to new records" and "Apply on record changes"
  checkboxes, and options for selecting which changes to recalculate on.
- For consistency, always represent empty RefList as None
- Fix up generated SchemaTypes to remember that values are encoded.

Included test cases for the main planned use cases:
- Auto-filled UUID column
- Data cleaning
- NOW() formula for record's last-updated timestamp.
- Updates that depend on other columns.

Test Plan: Added a browser test.

Reviewers: jarek

Reviewed By: jarek

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D2885

app/client/ui2018/menus.ts

@@ -1,6 +1,7 @@
 import {Command} from 'app/client/components/commands';
 import {NeedUpgradeError, reportError} from 'app/client/models/errors';
 import {colors, testId, vars} from 'app/client/ui2018/cssVars';
+import {cssSelectBtn} from 'app/client/ui2018/select';
 import {IconName} from 'app/client/ui2018/IconList';
 import {icon} from 'app/client/ui2018/icons';
 import {commonUrls} from 'app/common/gristUrls';
@@ -280,24 +281,6 @@ const cssSelectBtnContainer = styled('div', `
   width: 100%;
 `);
 
-const cssSelectBtn = styled('div', `
-  width: 100%;
-  height: 30px;
-  line-height: 16px;
-  background-color: white;
-  font-size: ${vars.mediumFontSize};
-  padding: 5px;
-  border: 1px solid ${colors.darkGrey};
-  color: ${colors.dark};
-  --icon-color: ${colors.dark};
-  border-radius: 3px;
-  cursor: pointer;
-  outline: none;
-  -webkit-appearance: none;
-  -moz-appearance: none;
-  display: flex;
-`);
-
 const cssSelectBtnLink = styled('div', `
   display: flex;
   align-items: center;