Add server plugin system (#1447)

* Add server plugin system

Plugins are optional modules loaded at runtime. Specification:
* Plugins are loaded from the server.plugins module (directory
  server/plugins)
* The import_plugins method is run as part of the initialization of the
  server module in __init__.py

* Add plugins to the EB build process

* Remove bit of dead code

* Respond to feedback from @bmccandless

server/test/test_plugins.py

@@ -0,0 +1,34 @@
+import os
+import shutil
+import unittest
+
+from server.common.utils import import_plugins
+from server.test import PROJECT_ROOT, random_string
+
+
+class TestPlugins(unittest.TestCase):
+    """ Test plugin import functionality """
+
+    plugins_dir = f"{PROJECT_ROOT}/server/test/plugins"
+    test_plugin_path = f"{plugins_dir}/foo.py"
+    secret = random_string(8)
+
+    @classmethod
+    def setUpClass(cls) -> None:
+        if not os.path.isdir(cls.plugins_dir):
+            os.mkdir(cls.plugins_dir)
+        with open(cls.test_plugin_path, "w") as fh:
+            fh.write(f'SECRET = "{cls.secret}"\n')
+
+    @classmethod
+    def tearDownClass(cls) -> None:
+        if os.path.isdir(cls.plugins_dir):
+            shutil.rmtree(cls.plugins_dir)
+
+    def test_import_plugins(self):
+        self.assertTrue(os.path.isfile(self.test_plugin_path))
+        loaded_modules = import_plugins("server.test.plugins")
+        # test that import plugins found the file
+        self.assertEqual(["server.test.plugins.foo"], [ele.__name__ for ele in loaded_modules])
+        # test that the module was properly executed
+        self.assertEqual(self.secret, loaded_modules[0].SECRET)