proxy: use wherever http.Client is, update mautrix

Added a new common.ConfigurableTransport interface which mediabrowser,
ombi, jellyseer, discord, telegram and matrix (i.e.
ThirdPartService/ContactMethodLinker) now all implement. proxies are
bound to them in main.go, Email is still a special case (but from the
previous commit, mailgun does use the proxy).

mautrix/go has been updated, and context.TODO()s stuck everywhere since
I still don't really comprehend why I should use them (FIXME literally).

discord.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"fmt"
+	"net/http"
 	"strings"
 	"time"
 
@@ -60,6 +61,11 @@ func newDiscordDaemon(app *appContext) (*DiscordDaemon, error) {
 	return dd, nil
 }
 
+// SetTransport sets the http.Transport to use for requests. Can be used to set a proxy.
+func (d *DiscordDaemon) SetTransport(t *http.Transport) {
+	d.bot.Client.Transport = t
+}
+
 // NewAuthToken generates an 8-character pin in the form "A1-2B-CD".
 func (d *DiscordDaemon) NewAuthToken() string {
 	pin := genAuthToken()