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)

package.json

@@ -68,6 +68,7 @@
         "foundation-sites": "git+https://github.com/steemit/foundation-sites.git#e8e32c715bbc4c822b80b555345f61337269ca78",
         "git-rev-sync": "1.9.1",
         "highcharts": "6.1.0",
+        "hivesigner": "^3.2.0",
         "humanize-number": "0.0.2",
         "imports-loader": "0.7.1",
         "intl": "1.2.5",