bug: mermaid sequenceDiagram incorrect spacing between arrows and messageText

### Is this a hackmd.io issue?

- [X] Yes, the bug happens on https://hackmd.io
- [ ] No. If it's a self-hosted CodiMD instance, please fire up the issue in the repo of CodiMD: https://github.com/hackmdio/codimd

### Read our Official tutorial

It's on https://hackmd.io/c/tutorials/

或是中文版 https://hackmd.io/c/tutorials-tw/

### What's the problem

#### Current behaviour
With the diagram
```mermaid
%%{init: {'securityLevel': 'strict', 'theme':'forest'}}%%

sequenceDiagram
autonumber
participant User
participant GHRepository
participant GHAction
participant CoCRepository
participant GHUpstreamPRQueue
User--xGHRepository: clone repository to local
activate User
activate GHRepository
User->>GHRepository: push veneer changes to GitHub from local
GHRepository--xGHAction: <
activate GHAction
GHAction->>GHAction: Generate FBC from veneer
GHAction->>GHAction: validate FBC
GHAction->>CoCRepository: Open CoC PR for FBC for User
deactivate GHAction
deactivate GHRepository
deactivate User
```
the rendering displays the messageText with an awkward spacing that makes it look like the messageText belongs to the previous arrow, with extra undesirable vertical spacing.

### Note: This diagram is rendered correctly by github, so please look below at the screen captures for the hackmd.io behavior ###

This appears to be related to [this mermaid issue discussion](https://github.com/mermaid-js/mermaid/issues/883), where user `knsv` comments on Jul 19, 2019 that it is likely because mermaid is initialized before dynamically-loaded fonts are finalized.

#### Screenshot


Provide steps to reproduce:
1. In an active hackmd, paste in the supplied mermaid block
2. see awkward spacing :(

(script and images from the note https://hackmd.io/EIWEwm28TZiwHUG25cmLBw)

#### Expected behaviour

(rendered via mermaid.live):

#### Environment

I use HackMD on:

**Desktop**
- OS: MacOS, linux, windows
- Browser: firefox
- Browser Version: 90.0.2

#### Additional context

Add any other context about the problem here.