PLT-6965 jira integration (plus plugin scaffolding) (#6918)

* plugin scaffolding / jira integration

* add vendored testify packages

* webhook fix

* don't change i18n ids

* support configuration watching

* add basic jira plugin configuration to admin console

* fix eslint errors

* fix another eslint warning

* polish

* undo unintentional config.json commit >:(

* test fix

* add jira plugin diagnostics, remove dm support, add bot tag, generate web-safe secrets

* rebase, implement requested changes

* requested changes

* remove tests and minimize makefile change

* add missing license headers

* add missing comma

* remove bad line from Makefile

app/plugin/api.go

@@ -0,0 +1,37 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package plugin
+
+import (
+	"net/http"
+
+	"github.com/gorilla/mux"
+	"github.com/mattermost/platform/model"
+)
+
+type API interface {
+	// Loads the plugin's configuration
+	LoadPluginConfiguration(dest interface{}) error
+
+	// The plugin's router
+	PluginRouter() *mux.Router
+
+	// Gets a team by its name
+	GetTeamByName(name string) (*model.Team, *model.AppError)
+
+	// Gets a user by its name
+	GetUserByName(name string) (*model.User, *model.AppError)
+
+	// Gets a channel by its name
+	GetChannelByName(teamId, name string) (*model.Channel, *model.AppError)
+
+	// Gets a direct message channel
+	GetDirectChannel(userId1, userId2 string) (*model.Channel, *model.AppError)
+
+	// Creates a post
+	CreatePost(post *model.Post, teamId string) (*model.Post, *model.AppError)
+
+	// Returns a localized string. If a request is given, its headers will be used to pick a locale.
+	I18n(id string, r *http.Request) string
+}