fix MarkdownV2 support in Telegram

[greenx]

Without this, MarkdownM2 simply does not work.
Because only constant is taken from tgbotapi, here I just assigned the desired value.
Although in tgbotapi I also sent MR.

[z3bra]

I gave it a go, and it works. However, this triggered a few issues for me when using MarkdownV2, and some messages were not relayed because the parsing failed:

time="2020-07-08T16:53:55+02:00" level=error msg="SendMessage failed: Bad Request: can't parse entities: Character '.' is reserved and must be escaped with the preceding '\'" func=handleMessage file="gateway/handlers.go:228" prefix=gateway
time="2020-07-08T16:56:18+02:00" level=error msg="SendMessage failed: Bad Request: can't parse entities: Character '(' is reserved and must be escaped with the preceding '\'" func=handleMessage file="gateway/handlers.go:228" prefix=gateway
time="2020-07-08T16:56:34+02:00" level=error msg="SendMessage failed: Bad Request: can't parse entities: Character ')' is reserved and must be escaped with the preceding '\'" func=handleMessage file="gateway/handlers.go:228" prefix=gateway
time="2020-07-08T17:04:51+02:00" level=error msg="SendMessage failed: Bad Request: can't parse entities: Character '!' is reserved and must be escaped with the preceding '\'" func=handleMessage file="gateway/handlers.go:228" prefix=gateway

This is probably expected though. It's just really annoying to loose messages because the "parsing failed".
The best solution to this would be to escape characters from the raw message before formatting it. But I'm not sure that'd fit every workflow…

[42wim]

Yes, they should be escaped like mentioned in https://core.telegram.org/bots/api#markdownv2-style
But it's fine to keep this for another PR.

[haykam821]

Relates to #1203 and #1225.