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_cxgtool.py

@@ -1,6 +1,4 @@
-import random
 import shutil
-import string
 import unittest
 
 import anndata
@@ -8,7 +6,7 @@ import anndata
 from server.common.data_locator import DataLocator
 from server.converters.cxgtool import write_cxg
 from server.data_cxg.cxg_adaptor import CxgAdaptor
-from server.test import PROJECT_ROOT, app_config
+from server.test import PROJECT_ROOT, app_config, random_string
 from server.test.test_datasets.fixtures import pbmc3k_colors
 
 
@@ -31,8 +29,8 @@ class TestCxgAdaptor(unittest.TestCase):
         self.assertEqual(data.get_colors(), {})
 
     def convert_pbmc3k(self, **kwargs):
-        random_string = "".join(random.choice(string.ascii_letters) for _ in range(8))
-        data_locator = f"/tmp/test_{random_string}.cxg"
+        rand_str = random_string(8)
+        data_locator = f"/tmp/test_{rand_str}.cxg"
         self.fixtures.append(data_locator)
         source_h5ad = anndata.read_h5ad(f"{PROJECT_ROOT}/example-dataset/pbmc3k.h5ad")
         write_cxg(adata=source_h5ad, container=data_locator, title="pbmc3k", **kwargs)