feat: AWS S3 + Digitalocean Spaces storage modules (#1015)

* Provide basic implementation of AWS S3 storage module

* Abstract S3 Compatible Storage Module logic

* Refactor `getFileExtension()` into the `page` object

* Add implementation for Digitalocean storage module

* Remove accidental `async`/`await` in S3 Storage Module

* Remove argument from the call to `page.getFileExtension()`

https://github.com/Requarks/wiki/pull/1015#discussion_r321990073

server/models/pages.js

@@ -147,6 +147,20 @@ module.exports = class Page extends Model {
     return pageHelper.injectPageMetadata(this)
   }
 
+  /**
+   * Get the page's file extension based on content type
+   */
+  getFileExtension() {
+    switch (this.contentType) {
+      case 'markdown':
+        return 'md'
+      case 'html':
+        return 'html'
+      default:
+        return 'txt'
+    }
+  }
+
   /**
    * Parse injected page metadata from raw content
    *