Local queries (#641)

* Read local queries from pgweb home directory
* Refactor local query functionality
* Allow picking local query in the query tab
* WIP
* Disable local query dropdown during execution
* Only allow local queries running in a single session mode
* Add middleware to enforce local query endpoint availability
* Fix query check
* Add query store tests
* Make query store errors portable
* Skip building specific tests on windows

data/lc_example1.sql

@@ -0,0 +1,2 @@
+-- pgweb: host="localhost"
+select 'foo'

data/lc_example2.sql

@@ -0,0 +1,5 @@
+-- pgweb: host="localhost"
+-- some comment
+-- pgweb: user="foo"
+
+select 'foo'

data/lc_invalid_meta.sql

@@ -0,0 +1,2 @@
+-- pgweb: host="localhost" mode="foo"
+select 'foo'

data/lc_no_meta.sql

@@ -0,0 +1 @@
+select 'foo'