Add pinned posts (#4217)

webapp/i18n/en.json

@@ -1670,6 +1670,7 @@
   "navbar.toggle1": "Toggle sidebar",
   "navbar.toggle2": "Toggle sidebar",
   "navbar.viewInfo": "View Info",
+  "navbar.viewPinnedPosts": "View Pinned Posts",
   "navbar_dropdown.about": "About Mattermost",
   "navbar_dropdown.accountSettings": "Account Settings",
   "navbar_dropdown.console": "System Console",
@@ -1726,6 +1727,9 @@
   "post_info.permalink": "Permalink",
   "post_info.reply": "Reply",
   "post_info.system": "System",
+  "post_info.pin": "Pin to channel",
+  "post_info.unpin": "Un-pin from channel",
+  "post_info.pinned": "Pinned",
   "post_message_view.edited": "(edited)",
   "posts_view.loadMore": "Load more messages",
   "posts_view.newMsg": "New Messages",
@@ -1778,11 +1782,14 @@
   "rhs_root.mobile.flag": "Flag",
   "rhs_root.mobile.unflag": "Unflag",
   "rhs_root.permalink": "Permalink",
+  "rhs_root.pin": "Pin to channel",
+  "rhs_root.unpin": "Un-pin from channel",
   "search_bar.search": "Search",
   "search_bar.usage": "<h4>Search Options</h4><ul><li><span>Use </span><b>\"quotation marks\"</b><span> to search for phrases</span></li><li><span>Use </span><b>from:</b><span> to find posts from specific users and </span><b>in:</b><span> to find posts in specific channels</span></li></ul>",
   "search_header.results": "Search Results",
   "search_header.title2": "Recent Mentions",
   "search_header.title3": "Flagged Posts",
+  "search_header.title4": "Pinned posts in {channelDisplayName}",
   "search_item.direct": "Direct Message (with {username})",
   "search_item.jump": "Jump",
   "search_results.because": "<ul><li>If you're searching a partial phrase (ex. searching \"rea\", looking for \"reach\" or \"reaction\"), append a * to your search term.</li><li>Two letter searches and common words like \"this\", \"a\" and \"is\" won't appear in search results due to excessive results returned.</li></ul>",
@@ -1792,6 +1799,9 @@
   "search_results.usageFlag2": "You can add a flag to messages and comments by clicking the ",
   "search_results.usageFlag3": " icon next to the timestamp.",
   "search_results.usageFlag4": "Flags are a way to mark messages for follow up. Your flags are personal, and cannot be seen by other users.",
+  "search_results.usagePin1": "There are no pinned messages yet.",
+  "search_results.usagePin2": "You can pin a message by clicking the \"Pin to channel\" option from the message's menu.",
+  "search_results.usagePin3": "Pinned messages are accessible by all channel members and are a way to mark messages for future reference.",
   "setting_item_max.cancel": "Cancel",
   "setting_item_max.save": "Save",
   "setting_item_min.edit": "Edit",