button text: Change "Log in with" to "Sign in with" everywhere.

Apple's Human Interface Guidelines doc on the "Sign in with Apple"
button [1] (to be added in an upcoming commit) is quite clear that,
for logging in, the text must say "Sign in with Apple", with that
exact capitalization and wording.

In order to keep the buttons consistent, change the others to match.

[1]: https://developer.apple.com/design/human-interface-guidelines/sign-in-with-apple/overview/buttons/.

Author: chrisbobbe

docs/howto/dev-server.md

@@ -202,7 +202,7 @@ This will be `http://ADDRESS:9991`, where `ADDRESS` is the address you
 identified in step 2.  (Be sure to type the `http://`.)
 
 This should get you the login screen!  Unless you're working on the login
-flow itself, tap "Log in with dev account"; then pick any user to log in as.
+flow itself, tap "Sign in with dev account"; then pick any user to log in as.
 
 If you need to work more closely with authentication systems, or if you need
 to use the [Zulip REST API][rest-api], which requires an API key, this

src/start/AuthScreen.js

@@ -257,7 +257,7 @@ class AuthScreen extends PureComponent<Props> {
               style={styles.halfMarginTop}
               secondary
               text={{
-                text: 'Log in with {method}',
+                text: 'Sign in with {method}',
                 values: { method: auth.displayName },
               }}
               Icon={auth.Icon}

static/translations/messages_en.json

@@ -34,7 +34,7 @@
   "Password": "Password",
   "Why not start the conversation?": "Why not start the conversation?",
   "Chat": "Chat",
-  "Log in with {method}": "Log in with {method}",
+  "Sign in with {method}": "Sign in with {method}",
   "Cannot connect to server": "Cannot connect to server",
   "Wrong email or password. Try again.": "Wrong email or password. Try again.",
   "Wrong username or password. Try again.": "Wrong username or password. Try again.",