(core) Add new Grist sign-up page

Summary:
Available at login.getgrist.com/signup, the new sign-up page
includes similar options available on the hosted Cognito sign-up
page, such as support for registering with Google. All previous
redirects to Cognito for sign-up should now redirect to the new
Grist sign-up page.

Login is still handled with the hosted Cognito login page, and there
is a link to go there from the new sign-up page.

Test Plan: Browser, project and server tests.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3249

app/client/ui2018/IconList.ts

@@ -30,6 +30,10 @@ export type IconName = "ChartArea" |
   "FieldText" |
   "FieldTextbox" |
   "FieldToggle" |
+  "LoginStreamline" |
+  "LoginUnify" |
+  "LoginVisualize" |
+  "GoogleLogo" |
   "GristLogo" |
   "ThumbPreview" |
   "BarcodeQR" |
@@ -143,6 +147,10 @@ export const IconList: IconName[] = ["ChartArea",
   "FieldText",
   "FieldTextbox",
   "FieldToggle",
+  "LoginStreamline",
+  "LoginUnify",
+  "LoginVisualize",
+  "GoogleLogo",
   "GristLogo",
   "ThumbPreview",
   "BarcodeQR",

app/client/ui2018/cssVars.ts

@@ -164,10 +164,12 @@ export const testId: TestId = makeTestId('test-');
 
 // Min width for normal screen layout (in px). Note: <768px is bootstrap's definition of small
 // screen (covers phones, including landscape, but not tablets).
+const largeScreenWidth = 992;
 const mediumScreenWidth = 768;
 const smallScreenWidth = 576;   // Anything below this is extra-small (e.g. portrait phones).
 
 // Fractional width for max-query follows https://getbootstrap.com/docs/4.0/layout/overview/#responsive-breakpoints
+export const mediaMedium = `(max-width: ${largeScreenWidth - 0.02}px)`;
 export const mediaSmall = `(max-width: ${mediumScreenWidth - 0.02}px)`;
 export const mediaNotSmall = `(min-width: ${mediumScreenWidth}px)`;
 export const mediaXSmall = `(max-width: ${smallScreenWidth - 0.02}px)`;