Experimental implementation for webapp plugins (#7185)

* Start of experimental implementation for webapp plugins

* Updates to webapp plugin architecture

* Update pluggable test

* Remove debug code

webapp/tests/plugins/__snapshots__/pluggable.test.jsx.snap

@@ -0,0 +1,111 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`plugins/Pluggable should match snapshot with no overridden component 1`] = `
+<IntlProvider>
+  <Pluggable
+    components={Object {}}
+    theme={Object {}}
+  >
+    <ProfilePopover
+      hasMention={false}
+      isRHS={false}
+      src="src"
+      theme={Object {}}
+      user={Object {}}
+    >
+      <Popover
+        bsClass="popover"
+        id="user-profile-popover"
+        placement="right"
+        theme={Object {}}
+        title="@undefined"
+      >
+        <div
+          className="popover right"
+          id="user-profile-popover"
+          role="tooltip"
+          style={
+            Object {
+              "display": "block",
+              "left": undefined,
+              "top": undefined,
+            }
+          }
+          theme={Object {}}
+        >
+          <div
+            className="arrow"
+            style={
+              Object {
+                "left": undefined,
+                "top": undefined,
+              }
+            }
+          />
+          <h3
+            className="popover-title"
+          >
+            @undefined
+          </h3>
+          <div
+            className="popover-content"
+          >
+            <img
+              className="user-popover__image"
+              height="128"
+              src="src"
+              width="128"
+            />
+            <div
+              className="popover__row first"
+              data-toggle="tooltip"
+            >
+              <a
+                className="text-nowrap text-lowercase user-popover__email"
+                href="#"
+                onClick={[Function]}
+              >
+                <i
+                  className="fa fa-paper-plane"
+                />
+                <FormattedMessage
+                  defaultMessage="Send Message"
+                  id="user_profile.send.dm"
+                  values={Object {}}
+                >
+                  <span>
+                    Send Message
+                  </span>
+                </FormattedMessage>
+              </a>
+            </div>
+          </div>
+        </div>
+      </Popover>
+    </ProfilePopover>
+  </Pluggable>
+</IntlProvider>
+`;
+
+exports[`plugins/Pluggable should match snapshot with overridden component 1`] = `
+<Pluggable
+  components={
+    Object {
+      "ProfilePopover": [Function],
+    }
+  }
+  theme={Object {}}
+>
+  <ProfilePopoverPlugin
+    hasMention={false}
+    isRHS={false}
+    src="src"
+    theme={Object {}}
+    user={Object {}}
+  >
+    <span>
+      ProfilePopoverPlugin
+    </span>
+  </ProfilePopoverPlugin>
+</Pluggable>
+`;