Website UI design (#4829)

**Fixed different issues** :
 
- Multiple CSS fixes: font-size, colors, margins, z-index ...
- Fixed hover on contributor avatars
- Added link to contributors in footer
- Made the year in the footer dynamic (2023 --> 2024)
- Added name of contributor in "Thank you" section of Contributor page
- Added footer in small screens
- Made Activity Log Responsive 
- Fixed bug in "saving issues to DB", title was null everywhere. I
needed to implement an "upsert" behaviour to update the existing
database on init

**To be noted :** 

There is the following bug on production happening on mobile when you
refresh a second time :

<img width="1440" alt="Screenshot 2024-04-05 at 01 30 58"
src="https://github.com/twentyhq/twenty/assets/102751374/b935b07a-63dc-463d-8dcb-070ad4ef6db0">


It seems to be related to the following issue on mdx :
[https://github.com/hashicorp/next-mdx-remote/issues/350](https://github.com/hashicorp/next-mdx-remote/issues/350)

I added the following code that fixed this bug for me in development
(this needs to be tested in production) :

```
const serialized = await serialize(content, {
    mdxOptions: {
      development: process.env.NODE_ENV === 'development',
    }
  })
```

---------

Co-authored-by: Ady Beraud <a.beraud96@gmail.com>

packages/twenty-website/src/app/contributors/[slug]/page.tsx

@@ -127,7 +127,7 @@ export default async function ({ params }: { params: { slug: string } }) {
             }[]
           }
         />
-        <ThankYou authorId={contributor.login} />
+        <ThankYou username={contributor.id} />
       </ContentContainer>
     </>
   );

packages/twenty-website/src/app/contributors/api/save-prs-to-db.tsx

@@ -15,6 +15,7 @@ export async function savePRsToDB(
     if (pr.author == null) {
       continue;
     }
+
     await insertMany(
       userModel,
       [
@@ -43,7 +44,7 @@ export async function savePRsToDB(
           authorId: pr.author.login,
         },
       ],
-      { onConflictKey: 'id' },
+      { onConflictKey: 'id', onConflictUpdateObject: { title: pr.title } },
     );
 
     for (const label of pr.labels.nodes) {

packages/twenty-website/src/app/contributors/page.tsx

@@ -47,9 +47,7 @@ const Contributors = async () => {
       <Background />
       <ContentContainer>
         <Header />
-        <div>
-          <AvatarGrid users={fitlerContributors as Contributor[]} />
-        </div>
+        <AvatarGrid users={fitlerContributors as Contributor[]} />
       </ContentContainer>
     </>
   );