Initial copy of files from the riot-web repo

None of this will work as it will need tweaking (at the very least
I've not copied the origin migrator because that's had long enough)
but these are files which already existed in their current state and
so don't need re-reviewing.

build/entitlements.mac.plist

@@ -0,0 +1,25 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+    <!-- Entitlements from electron-builder's defaults
+         (https://github.com/electron-userland/electron-builder/blob/master/packages/app-builder-lib/templates/entitlements.mac.plist)
+         nb. This does *not* include the app sandbox: at the time of adding this file,
+         we were using electron-builder 21.2.0 which does not have the sandbox entitlement.
+         Latest electron-builder does, but it appears to be causing issues:
+         (https://github.com/electron-userland/electron-builder/issues/4390)
+    -->
+    <key>com.apple.security.cs.allow-unsigned-executable-memory</key>
+    <true/>
+    <!-- https://github.com/electron-userland/electron-builder/issues/3940 -->
+    <key>com.apple.security.cs.disable-library-validation</key>
+    <true/>
+
+    <!-- Our own additional entitlements (we need to access the camera and
+        mic for VoIP calls -->
+    <key>com.apple.security.device.camera</key>
+    <true/>
+    <key>com.apple.security.device.audio-input</key>
+    <true/>
+</dict>
+</plist>

build/linux/after-install.tpl

@@ -0,0 +1,14 @@
+#!/bin/bash
+
+# Link to the binary
+ln -sf '/opt/${productFilename}/${executable}' '/usr/bin/${executable}'
+
+# SUID chrome-sandbox for Electron 5+
+# Remove this entire file (after-install.tpl) and remove the reference in
+# package.json once this change has been upstreamed so we go back to the copy
+# from upstream.
+# https://github.com/electron-userland/electron-builder/pull/4163
+chmod 4755 '/opt/${productFilename}/chrome-sandbox' || true
+
+update-mime-database /usr/share/mime || true
+update-desktop-database /usr/share/applications || true