(core) Comments

Summary:
First iteration for comments system for Grist.
- Comments are stored in a generic metatable `_grist_Cells`
- Each comment is connected to a particular cell (hence the generic name of the table)
- Access level works naturally for records stored in this table
-- User can add/read comments for cells he can see
-- User can't update/remove comments that he doesn't own, but he can delete them by removing cells (rows/columns)
-- Anonymous users can't see comments at all.
- Each comment can have replies (but replies can't have more replies)

Comments are hidden by default, they can be enabled by COMMENTS=true env variable.
Some things for follow-up
- Avatars, currently the user's profile image is not shown or retrieved from the server
- Virtual rendering for comments list in creator panel. Currently, there is a limit of 200 comments.

Test Plan: New and existing tests

Reviewers: georgegevoian, paulfitz

Reviewed By: georgegevoian

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D3509

app/client/ui2018/IconList.ts

@@ -39,6 +39,7 @@ export type IconName = "ChartArea" |
   "BarcodeQR" |
   "BarcodeQR2" |
   "CenterAlign" |
+  "Chat" |
   "Code" |
   "Collapse" |
   "Convert" |
@@ -75,6 +76,7 @@ export type IconName = "ChartArea" |
   "Lock" |
   "Log" |
   "Mail" |
+  "Message" |
   "Minus" |
   "MobileChat" |
   "MobileChat2" |
@@ -90,6 +92,7 @@ export type IconName = "ChartArea" |
   "Pivot" |
   "PivotLight" |
   "Plus" |
+  "Popup" |
   "Public" |
   "PublicColor" |
   "PublicFilled" |
@@ -165,6 +168,7 @@ export const IconList: IconName[] = ["ChartArea",
   "BarcodeQR",
   "BarcodeQR2",
   "CenterAlign",
+  "Chat",
   "Code",
   "Collapse",
   "Convert",
@@ -201,6 +205,7 @@ export const IconList: IconName[] = ["ChartArea",
   "Lock",
   "Log",
   "Mail",
+  "Message",
   "Minus",
   "MobileChat",
   "MobileChat2",
@@ -216,6 +221,7 @@ export const IconList: IconName[] = ["ChartArea",
   "Pivot",
   "PivotLight",
   "Plus",
+  "Popup",
   "Public",
   "PublicColor",
   "PublicFilled",

app/client/ui2018/cssVars.ts

@@ -50,6 +50,7 @@ export const colors = {
 
   light: new CustomProp('color-light', '#FFFFFF'),
   dark: new CustomProp('color-dark', '#262633'),
+  darkText: new CustomProp('color-dark-text', '#494949'),
   darkBg: new CustomProp('color-dark-bg', '#262633'),
   slate: new CustomProp('color-slate', '#929299'),