No Logo for Arweave in import/export settings

### 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

> Screenshot (optional)

Provide steps to reproduce:

1. Go to any hackmd document
2. Click on three settings dots
3. Scroll down to import export lists
4. See error: No logo for arweave

If possible, create a note with reproducible content and share the link with us.

e.g. note link: https://hackmd.io/features

#### Expected behaviour

Should have an image

#### Environment

I use HackMD on:

**Desktop**
- OS: [eg. macOS]
- Browser: [e.g. chrome, safari, firefox, opera]
- Browser Version: [71]

**Mobile**

- Device: [e.g. iPhone XS]
- OS: [e.g. iOS 12.1]
- Browser: [e.g. chrome, safari]
- Version: [e.g. 23]

#### Additional context