Improve default ToS

In an update of the Pleroma logo, the size of the logo increased from 38 × 50 to 250 × 250 pixels. After this change, the logo is shown with too large size in the default Terms of Services panel.

Now in this MR, the size of the logo is explicitly indicated like <img style="max-width: 100%; height: 50px; object-fit: contain;">.

Additionally redundant <p> and <br> are reduced in this MR.

Before this MR:

After this MR:

static/terms-of-service.html

 <h4>Terms of Service</h4>
 
 <p>This is a placeholder ToS. Edit <code>"/static/terms-of-service.html"</code> to make it fit the needs of your instance.</p>
-<img width="50px" height="50px" src="/static/logo.png" style="display: block; margin: auto;" />
+<img src="/static/logo.png" style="display: block; margin: auto; max-width: 100%; height: 50px; object-fit: contain;" />