[accessibility] Underline links in app

# Issue details

Accessibility requirement: never rely on colour alone to convey information, as some people may either:
- have contrasted themes (for instance yellow on blue: every foreground text, border, etc. becomes yellow, every background becomes blue), which make it impossible to determine functionality based on colour alone.
- have trouble seeing colours (for some forms of colour blindness).

The basic rule is: anything conveyed by colour should also be conveyed by another graphical means (eg. shape). Basically, for links the convention is usually to underline them.

### Duplicate?
no (searched for issue, does not seem to exist yet)

### Actual behaviour
- Open an article,
- Hyperlinks are just a different colour.

### Expected behaviour
- Open an article,
- Hyperlinks are a different colour **and** are underlined.

# Steps to reproduce the issue
1. Display any article.
2. See that links are not underlined.

# Environment details
(not really relevant here, I suspect, but here goes)
- **wallabag app version**: 2.4.3 (229)
- **wallabag app installation source** (e.g. Gplay, F-Droid, manual): unknown
- **Android OS version**: 9
- **Android ROM** (e.g. stock, LineageOS, SlimRom,…): stock
- **Android hardware**: irrelevant
- **wallabag server version**: 2.4.2 (app.wallabag.it
- **Do you have Two-Factor-Authentication enabled?**: no

# Your experience with wallabag Android app
Love it. 'nuff said :grin: