Add "Share article" feature

A new "shareCode" field is generated for each entry, and allows
unlogged users to access the entry through the /shared endpoint.
This feature is particularly useful to share articles from miniflux
to third-party users without having them to visit the original source.

The image proxy is disabled and special cache headers are proposed in
the shared page to avoid denial of service.

ui/share.go

@@ -0,0 +1,57 @@
+// Copyright 2017 Frédéric Guillot. All rights reserved.
+// Use of this source code is governed by the Apache 2.0
+// license that can be found in the LICENSE file.
+
+package ui // import "miniflux.app/ui"
+
+import (
+	"net/http"
+	"time"
+
+	"miniflux.app/http/request"
+	"miniflux.app/http/response"
+	"miniflux.app/http/response/html"
+	"miniflux.app/http/route"
+	"miniflux.app/storage"
+	"miniflux.app/ui/session"
+	"miniflux.app/ui/view"
+)
+
+func (h *handler) shareGenerate(w http.ResponseWriter, r *http.Request) {
+	entryID := request.RouteInt64Param(r, "entryID")
+	shareCode, err := h.store.GetEntryShareCode(request.UserID(r), entryID)
+	if err != nil {
+		html.ServerError(w, r, err)
+		return
+	}
+
+	html.Redirect(w, r, route.Path(h.router, "share", "shareCode", shareCode))
+}
+
+func (h *handler) sharePage(w http.ResponseWriter, r *http.Request) {
+	shareCode := request.RouteStringParam(r, "shareCode")
+	if shareCode == "" {
+		html.NotFound(w, r)
+		return
+	}
+
+	etag := shareCode
+	response.New(w, r).WithCaching(etag, 72*time.Hour, func(b *response.Builder) {
+		builder := storage.NewAnonymousQueryBuilder(h.store)
+		builder.WithShareCode(shareCode)
+
+		entry, err := builder.GetEntry()
+		if err != nil || entry == nil {
+			html.NotFound(w, r)
+			return
+		}
+
+		sess := session.New(h.store, request.SessionID(r))
+		view := view.New(h.tpl, r, sess)
+		view.Set("entry", entry)
+
+		b.WithHeader("Content-Type", "text/html; charset=utf-8")
+		b.WithBody(view.Render("entry"))
+		b.Write()
+	})
+}