Replace nodes/tags with contexts

The nodes/tags system has not been particularly successful, it's not as
intuitive as it could be, and in an attempt to be a single feature to address
many use cases, ends up addressing none of them particularly well.

This commit replaces nodes and tags with contexts.

Each job may define which context(s) the job may be associated with.
Laminar will only pop the job off the waiting queue when it can be assigned
to a context. A context defines an integer number of executors, which
represents how many runs can be simultaneously assigned to it. A context
may provide extra environment variables.

Essentially, a context replaces a node, and tags are gone. You just assign
jobs to contexts directly, and you can use a glob expression. This should be
more intuitive.

For grouping jobs in the WebUI, a separate mechanism called "groups" is provided.

src/context.h

@@ -0,0 +1,51 @@
+///
+/// Copyright 2015-2019 Oliver Giles
+///
+/// This file is part of Laminar
+///
+/// Laminar is free software: you can redistribute it and/or modify
+/// it under the terms of the GNU General Public License as published by
+/// the Free Software Foundation, either version 3 of the License, or
+/// (at your option) any later version.
+///
+/// Laminar is distributed in the hope that it will be useful,
+/// but WITHOUT ANY WARRANTY; without even the implied warranty of
+/// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+/// GNU General Public License for more details.
+///
+/// You should have received a copy of the GNU General Public License
+/// along with Laminar.  If not, see <http://www.gnu.org/licenses/>
+///
+#ifndef LAMINAR_CONTEXT_H_
+#define LAMINAR_CONTEXT_H_
+
+#include <fnmatch.h>
+#include <string>
+#include <set>
+class Run;
+
+// Represents a context within which a Run will be executed. Allows applying
+// a certain environment to a set of Jobs, or setting a limit on the number
+// of parallel Runs
+class Context {
+public:
+    Context() {}
+
+    std::string name;
+    int numExecutors;
+    int busyExecutors = 0;
+
+    bool canQueue(std::set<std::string>& patterns) {
+        if(busyExecutors >= numExecutors)
+            return false;
+
+        for(std::string pattern : patterns) {
+            if(fnmatch(pattern.c_str(), name.c_str(), FNM_EXTMATCH) == 0)
+                return true;
+        }
+        return false;
+    }
+};
+
+
+#endif // LAMINAR_CONTEXT_H_