HiveSigner integration

Description

Login and perform operations with HiveSigner.

For more details, please refer to https://peakd.com/hive-139531/@good-karma/imagehoster-hivesigner-and-condenser-bounty#@roomservice/re-good-karma-qd5qsb

The Merge Request contains almost the same change as condenser!74 (merged), except some minor differences.

Relevant Issues

Fixes: condenser#43 (closed)

config/default.json

@@ -7,7 +7,7 @@
         "directives": {
             "childSrc": "'self'",
             "connectSrc":
-                "'self' hive.blog https://api.hive.blog https://anyx.io api.blocktrades.us",
+                "'self' hive.blog https://api.hive.blog https://anyx.io api.blocktrades.us https://hivesigner.com",
             "defaultSrc": "'self'",
             "fontSrc": "data: fonts.gstatic.com",
             "frameAncestors": "'none'",